1. Field of the Invention
This invention relates to iron-silicon alloy powder magnetic cores used in noise filters and choke coils for switching power supplies and a process of manufacturing such powder magnetic cores.
2. Description of the Prior Art
The prior art includes the following process for manufacturing materials for powder magnetic cores used in magnetic noise suppression and choke coils. Specifically, the process is as follows: magnetic metal powders including pure iron, carbonyl iron, Fe-Ni alloy (hereafter referred to as Permalloy), or Fe-Si-Al alloy (hereafter referred to as Sendust) to which insulating binders such as sodium silicate or epoxy resin are added are compacted under pressure of 1-15 ton/cm.sup.2 and then heat-treated to relieve compression stresses (Japan Society of Powder and Powder Metallurgy, Magnetic Materials (1970), The Nikkan Kogyo Shimbun, Ltd.).
Pure iron powder cores are used in choke coils for switching power supplies at frequencies of 50 kHz or lower, transformers for ringing-choke-type power supply circuits, and noise suppressors in circuits in which low-frequency currents are superimposed.
Permalloy powder cores are used as cores for secondary-side smoothing chokes in switching power supplies in the frequency range of 100-150 kHz, and as noise-suppressors. Sendust powder cores can be used in the same frequency range as permalloy powder cores.
However, with recent requirements for severe control of high-frequency noise in electronic equipment, as well as for smaller and more compact equipment, powder cores of high permeability and low core loss are increasingly required. To manufacture powder cores, the powder particles are insulated by epoxy resin or sodium silicate to-avoid direct contact between powder particles and decrease eddy current .losses in the high-frequency region. Furthermore, pressing is used to increase density and obtain high permeability and low core loss.
In order to obtain high permeability, one needs to increase the packing density by high compacting pressure, but with conventional Sendust powder cores, the powder is extremely hard and resistant to plastic deformation, making high-pressure compacting difficult and markedly decreasing the life of dies.
Permalloy powder cores have higher permeability than pure iron powder cores and their high-frequency magnetic properties are excellent, but they are expensive and the adhesion of powder to insulating layer is insufficient so that the insulation between particles breaks down, markedly degrading magnetic properties in the high frequency region.